Powder metallurgy techniques, by which products are made by compacting metal powders into a mold and sintering, have become increasingly important in the production of products for, e.g., aerospace applications. Power metallurgy proves particularly useful for making parts of refractory metals that have such high melting points that conventional melting and casting is difficult. Powder metallurgy results in a finer and more uniform grain size, with a minimum of segregation and grain boundary precipitates.
Many methods have been used for producing the metal powders used in powder metallurgy including, among others, crushing, atomizing, condensation, reduction, precipation, electrodeposition and the characteristics of the powders depend, to a great extent, on the specific manufacturing process utilized. Perhaps the most important characteristic is the size of the individual particles made by the process. As the particles become smaller, the cumulative surface area of the particles increases very rapidly. This has an extremely important bearing on the properties of the powder, its behavior during processing into solid bodies, and the ultimate properties of these products Finer homogeneous (i.e. having an amorphous and/or microcrystalline structure) grains promote better strength, toughness and corrosion resistance. Further, ultrafine grain size (particles having a mean diameter of less than 125 microns) often make it possible to achieve superplastic behavior, which is of enormous benefit in near-net forming the material.
Another factor affecting the processing behavior of the metal powder and the final properties of the products made therewith is the shape of the individual particles. This, too, proves highly dependent upon the method of the producing the powder. Spherical powders are most desirable as they permit the maximum number of particles to fill a given volume.
One advantageous method of creating the metal powders for use in powder metallurgy is to melt a metal workpiece, create droplets of the molten metal, and rapidly solidify the molten droplets. Several processes are reviewed in Rapid Solidification Processing of Titanium Alloys, Journal of Metals, September 1983, namely: laser surface melting, electron-beam melting and splat quenching, laser melting/spin atomization, and ultrasonic gas atomization. Laser surface melting involves the self-quenching of a thin melted surface layer on a bulk material substrate by irradiating the surface of the material with a laser beam that transverses the surface at rates of between 1 and 50 cm per second to create melt depths of 10-1,000 microns. The molten droplets may be expelled by centrifugal force or by use of an ultrasonic gas jet having a high velocity (Mach 2-2.5), and a high frequency (80-100 kHz) to create inert gas pulses to rapidly atomize and solidify a stream of molten metal. Such techniques have resulted in homogeneously distributed, fine, incoherent dispersoids. However, because these processes involve rotating the feedstock at high angular velocities (up to 30,000 rpm), they are limited to materials that can be conventionally melted and formed into complex feedstock shapes. This, consequently, limits the different types of materials that can be made into powders, as well as limiting the amount of powder that can be produced from a given workpiece. Further, the melting of the material prior to the casting of the feedstock also adds contaminants. New powder applications require the manufacture of powders from alloying materials having widely different physical characteristics. New processes for making powders must also eliminate contamination while retaining powder homogeneity. Further, it is desirable to obtain higher yields of ultrafine (in the 25-50 micron range), spherical particles at higher rates of production, while obtaining a greater consistency and predictability in the production thereof.